1. Field of the Invention
The present invention relates to engine control and more particularly to the control of the burnt gas recirculation rate for a gasoline engine provided with an exhaust gas recirculation circuit (EGR circuit).
2. Description of the Prior Art
Downsizing gasoline engines currently appears to be the preferred option for reducing the consumption of fuel. In fact, this technology allows shifting the working points of the engine to zones of higher efficiency and thus to limit the pumping losses inherent in the operation of an internal-combustion engine. This type of engine then requires the presence of a compressor driven by a turbine arranged in the exhaust line. Such a device is used to improve air filling of the cylinder and to provide a torque equivalent to that of an engine of conventional displacement. It is thus possible to have the same performances of a conventional displacement engine while drastically reducing the consumption.
However, using such a technology greatly increases the risk of engine knocking. When the engine runs under full load conditions, the thermodynamic conditions in the combustion chamber may be detrimental to the stability of the mixture and generate auto-ignition thereof. This phenomenon can eventually greatly deteriorate the combustion chamber.
To solve this problem, the ignition advance is usually degraded. This option generates an increase in the gas temperature at the end of the combustion cycle and therefore over the entire exhaust line. Thus, in order to compensate for this phenomenon, the mixture is richened at the intake.
Such a method involves two drawbacks: which are increasing the engine consumption and furthermore, it deteriorates the efficiency of the catalyst arranged downstream from the exhaust manifold, which provides optimal conversion of the pollutants resulting from the combustion when the mixture is in stoichiometric proportions.
In this context, exhaust gas recirculation (EGR) from the exhaust to the intake is a promising option. Indeed, feeding burnt gas that has not reacted during combustion into a cylinder of the engine allows decreasing the overall combustion temperature and to limit engine knocking. The advantages of downsizing in terms of efficiency and consumption are thus preserved. Besides, introduction of burnt gas also allows reduction of the temperature of the exhaust gas and therefore limiting the impact thereof on the catalyst or the turbine.
However, such a strategy has a great influence on the overall running conditions of the engine. For example, the air mass trapped in the cylinder is smaller in an EGR configuration since burnt gas takes the place of fresh air in the cylinder. To operate under stoichiometric conditions, it is necessary to adjust the fuel loop to the air loop, and thus to control very precisely the amount of burnt gas in the cylinder. Besides, a high-performance control method is essential for torque transient management, notably at low loads where too high a proportion of burnt gas can extinguish the combustion.
To control the amount of burnt gas, current systems (described for example in patent applications FR-2,947,007 A1, EP-2,098,710 A1 and EP-0,485,089 A2) use an air flow detector, which has the drawback of being imprecise. The imprecisions of this detector do not allow optimum control of the composition of the gas in the intake manifold and thus operation under stoichiometric conditions, which generates imprecisions regarding engine control and influences the overall running conditions of the engine.
Control of the mixture composition at the intake is an essential component of the combustion control of supercharged gasoline engines.